Kerbal Space Program

You can tell Mechjeb ascent guidance to correct for terminal velocity and engine overheating or ignore those factors. There is also a seperate "engine" tab that has the same options - not sure if those are connected to the other autopilot maneuver options or what.

I thought for the first ~3k of ascent you shouldn't be doing much more then 160m/s - am I not recalling that correctly? Anything more then that and you are wasting fuel.

I tend to go full throttle to make sure we clear the pad and everything is under control and then throttle back as quickly as I can (if it is a rocket I have used a bunch I might launch at 50%).

When I fly my rockets that start getting time drag, it is easier to watch the ascent acceleration profile. Off the pad I always go 100% throttle until I reach 120m/s or my mainsails start to overheat. Only on very light rockets to I have to slow down due to speed. Most heavy lifters get to around 80-90 m/s. Dropping throttle to 3/4 lets the mailsail engines cool down and allows my acceleration to slowly creep up as I gain altitude.

It's a good rule to never exceed 250m/s until getting through the first blue bar of the Atmo indicator. MaxQ (maximum dynamic pressure) occurs right around 10k altitude. Most rockets tear themselves up from the dynamic load right about there. After reaching 12,000 it quickly drops off and it's usually safe to push the throttles open.

If you 'coast' through the last quarter of the first blue Atmo bar (~8k to ~11k) with almost no acceleration you will probably save those really far out rocket designs from ripping themselves apart.

I like to use SRBs on launch and never worry about air drag - the SRBs just chuck me over that portion

I do too, normally; but even with relatively small payloads, even four SRBs seem to be problematic. Granted, I'm just adding weight at that point but I think I'm going to focus on LOX engines since I can control thrust and use them more efficiently for orbital maneuvers and such.

I do too, normally; but even with relatively small payloads, even four SRBs seem to be problematic. Granted, I'm just adding weight at that point but I think I'm going to focus on LOX engines since I can control thrust and use them more efficiently for orbital maneuvers and such.

Maybe if you hadn't vanished when I was trying to show you how to launch

A good rocket design tip is not to add engines: add fuel, but don't bother with extra engines until your thrust-to-weight ratio gets down below 1.5 or so (if you can't be bothered working that out, don't add boosters until it won't lift off the pad :-) Too many engines is probably the most common design mistake to make. (SRBs are kind of free in this respect but for most rocket designs they actually produce too much thrust to be useful. Maybe good for getting something ultra heavy off the pad)

If you're really careful, you can get into orbit with way less fuel than you'd think. Best I've done flying manually was getting my Duna lander into Kerbin orbit (landercan + two of the smallest rockomax tanks + the poodle for an engine + a bunch of crap I stuck on to make it look cooler)

There's another way into orbit, though. It's a bit unreliable but I've achieved it more than once, apparently. This requires a story: in 1956, a design error caused the force of an underground nuclear test to be focused onto the manhole cover on top of the shaft the bomb was in. A high-speed camera caught the cover making a swift exit at 150,000 mph (about 6 times Earth's escape velocity), making it the fastest man-made object in history.

The Kerbals have evidently taken note. Recently, upon flying out to Jool for the second time, I decided to look at some of the debris that I found out there (waay out there: there was stuff out at 3 times Jool's orbit). It turned out not to be fuel tanks from the previous mission as I at first surmised. Nope. It turned out to be plane parts: wings, landing gear, control surfaces, you name it. Not from space planes, either, just ordinary planes that may have suffered some slight interaction with the landscape on Kerbin. I'm not sure what they use for jet fuel, but it has some kick to it when it explodes...

(Oh yes, the first mission to Jool? Turned out it had an unstable orbit and eventually got ejected from the system; perfect slightshot manoeuvre via Tylo putting it on a hyperbolic trajectory out of the solar system. I think a rescue mission might be out of the question...)

(SRBs are kind of free in this respect but for most rocket designs they actually produce too much thrust to be useful. Maybe good for getting something ultra heavy off the pad)

If you're good with the throttle, a decent set of SRBs (or an indecent one, if you use KW Rocketry) will save you a lot of fuel on the way up. They produce enough thrust to hit the speed ceiling in atmosphere, but every meter of delta-vee that they're pushing out is one more still in your LFE engines when you get up to where it can really matter.

I would like to see the game model air resistance, though. I'd like to see the SRB stacks blast off on a tower of flame and then hit the speed ceiling hard enough to overstress and rip the rocket apart.

I've adapted my launches. I keep an eye on the throttle and as I hit 150m/s, I throttle down in small increments; at about 10km in altitude I can throttle up to around 90% to punch through, but at that point I'm already gaining enough speed without having burned more fuel than necessary. A few informal tests have gotten me in to orbit with enough fuel for a few maneuvers but I need to get the hang of the 10km turn; I'm finding that I end up in a north-south orbit instead of an equatorial one which might make an eventual Mun-shot more difficult since I'm not on the right elliptical plane. Still damned fun though.

A few informal tests have gotten me in to orbit with enough fuel for a few maneuvers but I need to get the hang of the 10km turn; I'm finding that I end up in a north-south orbit instead of an equatorial one which might make an eventual Mun-shot more difficult since I'm not on the right elliptical plane. Still damned fun though.

Use your navball, you want to turn towards the "90" marker. Takes a bit of practice, but it's not terribly hard once you realize that you need to use the navball, and the navball only when turning (although a stable design makes that several times easier, since that means you don't have to mess around with the turn as much).

A few informal tests have gotten me in to orbit with enough fuel for a few maneuvers but I need to get the hang of the 10km turn; I'm finding that I end up in a north-south orbit instead of an equatorial one which might make an eventual Mun-shot more difficult since I'm not on the right elliptical plane. Still damned fun though.

Use your navball, you want to turn towards the "90" marker. Takes a bit of practice, but it's not terribly hard once you realize that you need to use the navball, and the navball only when turning (although a stable design makes that several times easier, since that means you don't have to mess around with the turn as much).

Gotcha. One issue I've had is forgetting to put a SAS module on my rockets, or having an inherently unstable setup that causes the rocket to spin. I'm going to try again tonight and see how it goes; if this works out and I can get stuff in to a stable orbit, I'll work on getting to the Mun next.

A few informal tests have gotten me in to orbit with enough fuel for a few maneuvers but I need to get the hang of the 10km turn; I'm finding that I end up in a north-south orbit instead of an equatorial one which might make an eventual Mun-shot more difficult since I'm not on the right elliptical plane. Still damned fun though.

Use your navball, you want to turn towards the "90" marker. Takes a bit of practice, but it's not terribly hard once you realize that you need to use the navball, and the navball only when turning (although a stable design makes that several times easier, since that means you don't have to mess around with the turn as much).

Gotcha. One issue I've had is forgetting to put a SAS module on my rockets, or having an inherently unstable setup that causes the rocket to spin. I'm going to try again tonight and see how it goes; if this works out and I can get stuff in to a stable orbit, I'll work on getting to the Mun next.

If you don't already know:

SAS: provides torque, and only uses itself to aim the ship.ASAS: provides no torque, and uses other sources of torque (the pod, gimbaling, control surfaces, etc) to steer the rocket/ship.

A few informal tests have gotten me in to orbit with enough fuel for a few maneuvers but I need to get the hang of the 10km turn; I'm finding that I end up in a north-south orbit instead of an equatorial one which might make an eventual Mun-shot more difficult since I'm not on the right elliptical plane. Still damned fun though.

Use your navball, you want to turn towards the "90" marker. Takes a bit of practice, but it's not terribly hard once you realize that you need to use the navball, and the navball only when turning (although a stable design makes that several times easier, since that means you don't have to mess around with the turn as much).

Gotcha. One issue I've had is forgetting to put a SAS module on my rockets, or having an inherently unstable setup that causes the rocket to spin. I'm going to try again tonight and see how it goes; if this works out and I can get stuff in to a stable orbit, I'll work on getting to the Mun next.

If you don't already know:

SAS: provides torque, and only uses itself to aim the ship.ASAS: provides no torque, and uses other sources of torque (the pod, gimbaling, control surfaces, etc) to steer the rocket/ship.

What do you recommend? I've been slapping a SAS on my ships without paying much attention to what it does, so I think that's part of the problem with the stability of my ships.

I've noticed that if you can't hold the ship steady yourself, any SAS/ASAS probably isn't going to help much. It's more of a correct minor issues type of thing than a magical stability creator.

Typically, if I have something that is borderline stable, or just a nuisance to fly, I'll make sure to slap on an ASAS module that will be ejected with whatever stage ends up being the first stage ejected after leaving the atmosphere. The modules seem of limited value in no atmosphere.

If you're having flying issues, it's usually more related to your design than the (A)SAS modules, I think. Then again, I am no expert. I haven't advanced beyond building a little mun base, one of the modules of which is lying on its side instead of its legs.

I've noticed that if you can't hold the ship steady yourself, any SAS/ASAS probably isn't going to help much. It's more of a correct minor issues type of thing than a magical stability creator.

Typically, if I have something that is borderline stable, or just a nuisance to fly, I'll make sure to slap on an ASAS module that will be ejected with whatever stage ends up being the first stage ejected after leaving the atmosphere. The modules seem of limited value in no atmosphere.

If you're having flying issues, it's usually more related to your design than the (A)SAS modules, I think. Then again, I am no expert. I haven't advanced beyond building a little mun base, one of the modules of which is lying on its side instead of its legs.

That makes sense. I think the stability issues stem from the base rocket I've been using: a central stage with three rockets attached via decouplers. I'll add an ASAS and see if that helps keep things stable during ascent.

ASAS is pretty useful during docking and landing on, e.g., Minmus. Turning on ASAS (while pointed straight up) will keep your lander pointing up if it bounces a bit on the first landing, like if you still had some m/s of drift.

ASAS during docking is good for lower/faster orbits where over the course of 20 seconds or so a craft may rotate slightly. That, and locking your roll if you care about lining up on the docking axis. Just turn it off when the docking magnet grabs on.

That makes sense. I think the stability issues stem from the base rocket I've been using: a central stage with three rockets attached via decouplers. I'll add an ASAS and see if that helps keep things stable during ascent.

Did you use symmetry to add the 3 side rockets?

Also, you can turn off gimbaling of the side engines and that may help it stay more stable.

What do you recommend? I've been slapping a SAS on my ships without paying much attention to what it does, so I think that's part of the problem with the stability of my ships.

ASAS for ascent stages. It can use RCS or control surfaces to keep your ship on course, as well as by gimballing the rocket motors. This gives far more force than the SAS modules, which is what you want for the heavy ascent stages. (Aerodynamic control surfaces are probably the most weight-efficient way to add extra control in the lower atmosphere)

If you're flipping over, though, it probably won't fix the problem: it's mainly useful for stopping drift. What's probably happening is that you've got a wobbly rocket and it flips over because the thrust goes off center. You need struts to fix that problem :-) I think what happens is that when the air gets thin, it stops pushing down on the parts, which causes them to move: if they get a bit off center, then you get backflips. Fuel weight distribution can be an issue too, sometimes.

If you're using the tri-couplers, note that it's easy to accidentally stick a fuel tank to the side of another tank instead of to the coupler if you aren't using symmetry: this will produce an unstable rocket. (You'll also want to strut the things together at the bottom: if you look closely, you'll see they wobble quite a lot)

Duodecimal wrote:

ASAS is pretty useful during docking and landing on, e.g., Minmus.

I have to disagree here, though: this is where you want standard SAS. The problem with ASAS is that it will use your RCS to keep your orientation, which will change your ship's lateral velocity, putting you off course and making things difficult (especially for docking, though it's also a good way to tip over while landing).

That makes sense. I think the stability issues stem from the base rocket I've been using: a central stage with three rockets attached via decouplers. I'll add an ASAS and see if that helps keep things stable during ascent.

Did you use symmetry to add the 3 side rockets?

Also, you can turn off gimbaling of the side engines and that may help it stay more stable.

I have to disagree here, though: this is where you want standard SAS. The problem with ASAS is that it will use your RCS to keep your orientation, which will change your ship's lateral velocity, putting you off course and making things difficult (especially for docking, though it's also a good way to tip over while landing).

Ah ... in my case I was landing a really big, heavy Kethane refinery, and while it did have a set of four larger (KW) multidirectional thrusters on the core, the bulk of the RCS were twelve mono-thrusters pointing straight down, three on each leg construct.

So blasts of RCS weren't going to send it tumbling. A small probe on the other hand, no I wouldn't have done that, and yes an SAS would have sufficed

I present Jeb! the Kerbin escape capable solid fuel ship. 912 parts, 99 srbs, 1 pod, 6 winglets, 2 decouplers, and 804 struts! It brings my PC to its knees like the early alphas did when you had around 100 parts, but it doesn't blow up and doesn't fall apart. If you're a good driver, you could hit the mun.

SRB efficiency in most mods (and base game) is 200-250 Isp. Considering that a single FL-T400 and LV-T30 has an Isp of around 350, the SRB would need to be very light in order to justify adding it.

So I put together a simple command pod and both propulsion types.

The liquid fuel booster test was a launch mass of 4.2 tons and did a fuel-optimal ascent to 28,083 metres.

The SRB booster test was a launch mass of 4.55 tons and did a non-optimal ascent to 20,713 metres.

Given that the SRB underperformed due to it being non-throttlable, the test had to be reproduced such that neither engine exceeded terminal velocity on ascent, so neither was perfectly fuel optimal but this better considers real-game flights, as very few are fast enough at low altitudes to be optimal. This was done by adding two FL-T800 fuel tanks as a boilerplate second stage (unpowered). Being constant, it would not affect comparative results.

Worth noting is that a nearly exact launch mass match outperformed the SRB by over a third. Using big SRBs is, then, nearly indefensible. They're only any good for an underpowered rocket needing extra oomph off the pad, as they have quite impressive TWRs. I personally use the small ones to get a large rocket up to speed off the pad so it can do a more fuel-optimal ascent. This can make 10% more mass to orbit.

In all three test scenarios, the SRB performed best when it was only being asked to have high thrust. For anything else, it fell short enormously, being limp, weak and anti-Jebian.

Jeb approves of adding SRBs, but only to a worthy rocket.

If you still have any SRBs attached at 4,000 metres, you've done it wrong.

What do you recommend? I've been slapping a SAS on my ships without paying much attention to what it does, so I think that's part of the problem with the stability of my ships.

ASAS for ascent stages. It can use RCS or control surfaces to keep your ship on course, as well as by gimballing the rocket motors. This gives far more force than the SAS modules, which is what you want for the heavy ascent stages. (Aerodynamic control surfaces are probably the most weight-efficient way to add extra control in the lower atmosphere)

If you're flipping over, though, it probably won't fix the problem: it's mainly useful for stopping drift. What's probably happening is that you've got a wobbly rocket and it flips over because the thrust goes off center. You need struts to fix that problem :-) I think what happens is that when the air gets thin, it stops pushing down on the parts, which causes them to move: if they get a bit off center, then you get backflips. Fuel weight distribution can be an issue too, sometimes.

If you're using the tri-couplers, note that it's easy to accidentally stick a fuel tank to the side of another tank instead of to the coupler if you aren't using symmetry: this will produce an unstable rocket. (You'll also want to strut the things together at the bottom: if you look closely, you'll see they wobble quite a lot)

Duodecimal wrote:

ASAS is pretty useful during docking and landing on, e.g., Minmus.

I have to disagree here, though: this is where you want standard SAS. The problem with ASAS is that it will use your RCS to keep your orientation, which will change your ship's lateral velocity, putting you off course and making things difficult (especially for docking, though it's also a good way to tip over while landing).

That's how I normally play it too. ASAS for launch, SAS for everything else. Although, after comments on this thread my 'Kerbin' class orbital shuttles do include an ASAS module (used sparingly to avoid using too much monopropellant). I find that SAS is normally enough for attitude control and adjustment about all three axes, although it has to be said that my heavier station components do tend to steer like a pig on roller skates. Cautious, manual use of RCS helps there.

With regard to flipping over, you've maybe done this already but it's worth checking the centre of mass and centre of thrust for different sections of your rocket as you build it. For a stable design, the two should be as close together as possible

I've had a couple of designs, where a big heavy lower stage was masking balance problems further up the stack. Flew fine to begin with but the CoM of mass was far too high once I jettisoned the lower stage, causing the rest of the vehicle to flip.

I have to disagree here, though: this is where you want standard SAS. The problem with ASAS is that it will use your RCS to keep your orientation, which will change your ship's lateral velocity, putting you off course and making things difficult (especially for docking, though it's also a good way to tip over while landing).

That's how I normally play it too. ASAS for launch, SAS for everything else. Although, after comments on this thread my 'Kerbin' class orbital shuttles do include an ASAS module (used sparingly to avoid using too much monopropellant). I find that SAS is normally enough for attitude control and adjustment about all three axes, although it has to be said that my heavier station components do tend to steer like a pig on roller skates. Cautious, manual use of RCS helps there.

With regard to flipping over, you've maybe done this already but it's worth checking the centre of mass and centre of thrust for different sections of your rocket as you build it. For a stable design, the two should be as close together as possible

I've had a couple of designs, where a big heavy lower stage was masking balance problems further up the stack. Flew fine to begin with but the CoM of mass was far too high once I jettisoned the lower stage, causing the rest of the vehicle to flip.[/quote]

If I have both ASAS and RCS, I just make sure never to enable them both at the same time. ASAS for holding a course, RCS for maneuvers with manual controls.

After numerous attempts... I finally managed to once again land a Kerbal on the Mun!!!

Spoiler: show

He's so happy, I don't know how to break the news.....

Not even the mobile light tower rovers are functional and fell over. Serious design flaw.

That reminds me of the mission just prior to my first successful Mun landing. That prior mission touched down on the Mun just fine. Then I extended the ladder so that the crew could go for a walk, only to discover that I had put the extendable ladder too low on the hull. Ladder extended down below the level of the feet, dug into the ground, and promptly toppled the ship over.

Right then. Three more Kerbals marooned in the service of Science. Munlander 3.1 had the ladder mount moved somewhat higher up on the chassis.